Yellow-Colored Electro-Optic Crystals as Intense Terahertz Wave Sources

This study presents newly developed yellow-colored organic electro-optic crystals to provide high terahertz (THz) wave generation efficiency. Compared with currently existing red- or orange-colored electro-optic crystals, which are used for most benchmark organic THz sources, yellow-colored crystals have additional superior advantages for THz wave generation, e.g., higher transparency in the visible wavelength range with accompanying different phase-matching possibilities. The new yellow-colored crystals consist of a highly nonlinear optical 4-(4-hydroxystyryl)-1-methylpyridinium (OHP) cation, with a relatively short wavelength of maximal absorption at 390 nm in solution, and various halogen-substituted benzenesulfonate anions, with strong secondary-bonding ability. OHP 4-chlorobenzenesulfonate (OHP-CBS) crystals exhibit large off-resonant macroscopic optical nonlinearity and high transparency, with a cut-off wavelength for solid-state absorption near 490 nm. OHP-CBS crystals provide excellent THz wave generation characteristics based on optical rectification. A 0.53 mm thick OHP-CBS crystal delivers approximate to 27 times higher optical-to-THz conversion efficiency and a much broader spectrum bandwidth compared with the standard 1.0 mm thick ZnTe at 1300 nm pumping. Particularly, compared with a benchmark organic quinolinium crystal with a similar thickness of 0.55 mm, OHP-CBS crystals exhibit 1.7 times higher optical-to-THz conversion efficiency, and show a significantly different THz spectral shape.